In a typical ink jet printer using a multi-nozzle head, digital signals as to each of four colors (i.e., red, green, blue and black) regarding an image are processed in a manner so that the multi-nozzle head forms a printed color image on a recorder medium, such as paper or transparencies.
Indeed, ink jet printing has become recognized as a prominent contender in the digitally controlled, electronic printing arena because, e.g., of its non-impact, low-noise characteristics, its use of plain paper and its avoidance of toner transfers and fixing. Ink jet printing mechanisms can be categorized as either continuous ink jet or drop-on-demand ink jet. U.S. Pat. No. 3,946,398, which issued to Kyser et al. in 1970, discloses a drop-on-demand ink jet printer which applies a high voltage to a piezoelectric crystal, causing the crystal to bend, applying pressure on an ink reservoir and jetting drops on demand. Other types of piezoelectric drop-on-demand printers utilize piezoelectric crystals in push mode, shear mode, and squeeze mode. Piezoelectric drop-on-demand printers have achieved commercial success at image resolutions up to 720 dpi for home and office printers. However, piezoelectric printing mechanisms usually require complex high voltage drive circuitry and bulky piezoelectric crystal arrays, which are disadvantageous in regard to manufacturability and performance.
Great Britain Pat No. 2,007,162, which issued to Endo et al. in 1979, discloses an electrothermal drop-on-demand ink jet printer which applies a power pulse to an electrothermal heater which is in thermal contact with water based ink in a nozzle. A small quantity of ink rapidly evaporates, forming a bubble which cause drops of ink to be ejected from small apertures along the edge of the heater substrate. This technology is known as Bubblejet.TM. (trademark of Canon K.K. of Japan).
U.S. Pat. No. 4,490,728, which issued to Vaught et al. in 1982, discloses an electrothermal drop ejection system which also operates by bubble formation to eject drops in a direction normal to the plane of the heater substrate. As used herein, the term "thermal ink jet" is used to refer to both this system and system commonly known as Bubblejet.TM..
Thermal ink jet printing typically requires a heater energy of approximately 20 .mu.J over a period of approximately 2 .mu.sec to heat the ink to a temperature between 280.degree. C. and 400.degree. C. to cause rapid, homogeneous formation of a bubble. The rapid bubble formation provides the momentum for drop ejection. The collapse of the bubble causes a tremendous pressure pulse on the thin film heater materials due to the implosion of the bubble. The high temperatures needed necessitates the use of special inks, complicates the driver electronics, and precipitates deterioration of heater elements. The 10 Watt active power consumption of each heater is one of many factors preventing the manufacture of low cost high speed pagewidth printheads.
U.S. Pat. No. 4,275,290, which issued to Cielo et al., discloses a liquid ink printing system in which ink is supplied to a reservoir at a predetermined pressure and retained in orifices by surface tension until the surface tension is reduced by heat from an electrically energized resistive heater, which causes ink to issue from the orifice and to thereby contact a paper receiver. This system requires that the ink be designed so as to exhibit a change, preferably large, in surface tension with temperature. The paper receiver must also be in close proximity to the orifice in order to separate the drop from the orifice.
U.S. Pat. No. 4,166,277, which also issued to Cielo et al., discloses a related liquid ink printing system in which ink is supplied to a reservoir at a predetermined pressure and retained in orifices by surface tension. The surface tension is overcome by the electrostatic force produced by a voltage applied to one or more electrodes which lie in an array above the ink orifices, causing ink to be ejected from selected orifices and to contact a paper receiver. The extent of ejection is claimed to be very small in the above Cielo patents, as opposed to an "ink jet", contact with the paper being the primary means of printing an ink drop. This system is disadvantageous, in that a plurality of high voltages must be controlled and communicated to the electrode array. Also, the electric fields between neighboring electrodes interfere with one another. Further, the fields required are larger than desired to prevent arcing, and the variable characteristics of the paper receiver such as thickness or dampness can cause the applied field to vary.
In U.S. Pat. No. 4,751,531, which issued to Saito, a heater is located below the meniscus of ink contained between two opposing walls. The heater causes, in conjunction with an electrostatic field applied by an electrode located near the heater, the ejection of an ink drop. There are a plurality of heater/electrode pairs, but there is no orifice array. The force on the ink causing drop ejection is produced by the electric field, but this force is alone insufficient to cause drop ejection. That is, the heat from the heater is also required to reduce either the viscous drag and/or the surface tension of the ink in the vicinity of the heater before the electric field force is sufficient to cause drop ejection. The use of an electrostatic force alone requires high voltages. This system is thus disadvantageous in that a plurality of high voltages must be controlled and communicated to the electrode array. Also the lack of an orifice array reduces the density and controllability of ejected drops.
Each of the above-described ink jet printing systems has advantages and disadvantages. However, there remains a widely recognized need for an improved ink jet printing approach, providing advantages for example, as to cost, speed, quality, reliability, power usage, simplicity of construction and operation, durability and consumables.
Commonly assigned U.S. patent application Ser. No. 08/750,438 entitled A LIQUID INK PRINTING APPARATUS AND SYSTEM filed in the name of Kia Silverbrook on Dec. 3, 1996, discloses a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with drop size and placement accuracy, attainable printing speeds, power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks. Silverbrook provides a drop-on-demand printing mechanism wherein the means of selecting drops to be printed produces a difference in position between selected drops and drops which are not selected, but which is insufficient to cause the ink drops to overcome the ink surface tension and separate from the body of ink, and wherein an additional means is provided to cause separation of said selected drops from said body of ink. Several drop separation techniques are disclosed by Silverbrook, the following table entitled "Drop separation means" shows some of the possible methods for separating selected drops from the body of ink, and ensuring that the selected drops form dots on the printing medium. The drop separation means discriminates between selected drops and un-selected drops to ensure that un-selected drops do not form dots on the printing medium.
Drop separation means Means Advantage Limitation 1. Electrostatic Can print on rough Requires high voltage power attraction surfaces, simple supply implementation 2. AC electric field Higher field strength is Requires high voltage AC possible than electrostatic, power supply synchronized. operating margins can be to drop ejection phase. increased, ink pressure Multiple drop phase reduced, and dust operation is difficuit accumulation is reduced 3. Proximity Very small spot sizes can Requires print maximum to be (printhead in close be achieved. Very low very close to printhead proximity to, but not power dissipation. High surface, not suitable for touching, recording drop position accuracy rough print media, usually medium) requires transfer roller or 4. Transfer Proximity Very small spot sizes can Not compact due to size of (print-head is in close be achieved, very low transfer roller or transfer proximity to a power dissipation, high belt. transfer roller or belt accuracy, can print on rough paper 5. Proximity with Useful for hot melt inks Requires print medium to be oscillating ink using viscosity reduction very close to printhead pressure drop selection method, surface, not suitable for reduces possibility of rough print media. Requires nozzle clogging, can use ink pressure oscillation pigments instead of dyes apparatus 6. Magnetic Can print on rough. Requires uniform high attraction surfaces. Low power if magnetic field strength, permanent magnets are requires magnetic ink used
Silverbrook discloses a liquid printing system that affords significant improvements toward overcoming the prior art problems associated with drop size and placement accuracy, attainable printing speeds, power usage, durability, thermal stresses, other printer performance characteristics, manufacturability, and characteristics of useful inks. Silverbrook discloses a single microscopic nozzle tip having pressurized ink extending from the nozzle, which is formed from silicon dioxide layers with a heater and a nozzle tip. The nozzle tip is passivated with silicon nitride. The "Silverbrook" technique provides for low power consumption, high speed, and page-wide printing. In such ink jet printheads, the energy barrier for ejecting an ink droplet is reduced by reducing the surface tension of the ink solution. The ink solution in an ink reservoir is under a static pressure so that an ink meniscus is bulged outward at a nozzle outlet. For each selected nozzle, a voltage pulse is applied to a ring-shaped resistor. The heating of the resistor by the electric pulse reduces the surface tension of the ink solution in the vicinity of the rim of the nozzle. The heated ink solution is pushed outward by the static pressure. The interplay between the surface tension reduction by heating and the static pressure begins to dominate, and finally ejects the ink droplet to a receiver media. The separation of the droplet from the nozzle can be assisted by a static electric field applied that attracts the ink droplet toward the receiving media.
In other words, such an ink jet printer as described immediately hereinabove includes a multiplicity of nozzles having orifices opening toward the recorder medium. An ink droplet in each nozzle is under a predetermined static back-pressure in order to propel the ink droplet onto the recorder medium. However, before the ink droplet is propelled toward the recorder medium, it is initially restrained or held in the orifice by surface tension even though the ink droplet is under static back-pressure. This results in an ink meniscus bulging outwardly at the nozzle orifice without leaving the orifice. This is so because, by design, the back-pressure is initially insufficient to overcome the ink droplet's surface tension. Therefore, in order to print on the recorder medium, the surface tension of the ink droplet is decreased, so that the ink droplet is released from the nozzle orifice and propelled onto the recorder medium by the previously mentioned back-pressure. To decrease surface tension, a voltage pulse is applied to an electrical resistance heater that is located inside the nozzle and that is therefore in heat transfer communication with the ink droplet. Heating of the resistance heater by the voltage pulse heats the ink droplet, thereby reducing the surface tension of the ink droplet. Of course, the static back-pressure acting on the ink droplet coacts with the simultaneous decrease in surface tension to eject the ink droplet from the orifice and propel it onto the recorder medium.
However, ink jet printers may produce non-uniform print density with respect to the image deposited on the recorder medium. Such non-uniform print density may be visible as so-called "banding". "Banding" is evinced, for example, by repeated variations in the print density caused by delineations in individual dot rows comprising the output image. Thus, "banding" can appear as light or dark streaks or lines within a printed area. "Banding" is influenced by factors such as ink drop volume variations, print head carriage motion anomalies, electrical resistance variation of the heaters, and/or the presence of damaged nozzles.
One important factor producing "banding" is variability in the nozzle orifice diameter caused by variations in the manufacturing process used to make the nozzles constituting the print head. Even small variations between nozzles of a print head may lead to visible "banding". More specifically, when the ink droplet is pushed outwardly during ejection from the nozzle, the moving ink droplet must overcome flow resistance caused by the nozzle's flow channel and also flow resistance caused by the nozzle's orifice. Therefore, the ejection speed of the droplet is strongly dependent on the flow resistance or drag force exerted by the nozzle's flow channel and the nozzle's orifice. Nozzle diameter affects flow resistance or drag force and therefore affects the amount of ink ejected from the nozzles. Moreover, nozzle diameter also affects the meniscus shape of the ink at the nozzle's orifice, which in turn affects droplet volume and ejection rate. In addition, heater electrical resistance can vary among nozzles due to slight variations in the composition of the material comprising the electric resistance heaters disposed in the nozzles. Variations in electrical resistance among nozzles causes variations in the amount and ejection speed of the ink thereby leading to variations in print density. All the afore mentioned factors negatively affect print density and invite "banding". Therefore, a problem in the art is non-uniform print density due to the presence of physical variations among the print nozzles, such as variations in nozzle diameter and electrical resistance.
Techniques specifically addressing the problem of non-uniform print density are known. One such technique is disclosed in U.S. Pat. No. 5,038,208 titled "Image Forming Apparatus With A Function For Correcting Recording Density Unevenness" issued Aug. 6, 1991 in the name of Hiroyuki Ichikawa This patent discloses memory means for storing data corresponding to image forming characteristics (i.e., print density) of each nozzle of multi-nozzle print heads, and a corrector means for correcting the image forming signals based on the data stored in the memory means. However, this patent does not appear to disclose an efficient and cost effective solution to the problem of non-uniform print density or "banding". For example, the Ichikawa patent discloses that image processing is required for correcting density non-uniformities for each input image file. That is, image processing is required for each and every input image for which output density correction is desired. Correcting density non-uniformities for each input image file is undesirable because it is time consuming. Also, this patent discloses that modulation in the output code value is made at a relatively limited number of discrete levels for halftoned images at a typical printing resolution (i.e., 600 dots per inch). However, printing at discrete levels may not eliminate visual printing defects, such as "banding".
Therefore, what has long been needed is a suitable imaging apparatus and method for providing images of uniform print density, so that printing non-uniformities, such banding, are avoided.